This invention relates generally to sensors for locating hidden objects and more particularly to sensors for locating wall studs.
A common problem faced by anyone attempting to hang a picture or cabinet is how to precisely locate between-wall studs so a sturdy hook may be attached or clearance may be provided for the cabinet. Since wall studs are usually covered by sheetrock or wallboard and finished-off, stud location is not visible. A similar problem arises when hanging plants and lamps from the ceiling, or when nailing down squeaky floorboards and stair steps.
General methods for locating studs and joists include tapping with a hammer, searching for nails with a magnetic compass, and random piercing with a nail. Hammer tapping and magnetic compass searching are unreliable and time-consuming, and random piercing is destructive. Once a nail is located, it may be off-center. Also, the stud may be warped, making it impossible to deduce accurate stud location at any distance from the nail.
These primitive methods were vastly advanced when an electronic wall stud sensor became commercially available about ten years ago. The user places the sensor flat against the wall and scans it laterally across the extent of the wall. When it passes over a stud, a vertical series of LEDs light to indicate the presence of the stud behind the wall. The sensor is based on dielectric density sensing. U.S. Pat. No. 4,099,118 describes a portable electronic wall stud sensor having capacitor plates and circuitry for detecting changes in the capacitive charge due to changes in the dielectric constant in the wall adjacent the sensor. U.S. Pat. No. 4,464,622 describes a similar capacitive sensor with calibration means and means for detecting an AC line in the wall.
Dielectric density sensing has limitations. If a small air gap forms between the sensor and the wall, the device becomes inoperative due to the substantial change in density adjacent the two sensing plates that are internal to the unit. It is therefore difficult or impossible to locate studs on rough or highly textured surfaces.
Another limitation is that stud detection is directly affected by the dielectric constant of the intervening wall material. Sheetrock, plywood, particle board, and dense hardwoods vary in dielectric constant to such an extent that a dielectric sensor generally only works on sheetrock and not on plywood walls, wood floors, stair steps, furniture or cabinetry.